74G THE PEINCIPLES OF SCIENCE. [citap. 



of the law of gravity shou'S that all such disturbances are 

 essentially periodic, so that after the lapse of millions of 

 years the" planets will return to the same relative positions, 

 and a new cycle of disturbances m ill then connnence. 



As other branches of science progress, we seem to gain 

 assurance that no great alteration of the world's condition 

 is to be expected. Conflict with a comet has long been the 

 cause of fear, but now it is credibly asserted that we have 

 passed through a comet's tail without the fact being known 

 at the time, or manifested by any more serious a phenomenon 

 than a slight luminosity of the sky. More recently still 

 the earth is said to have touched the comet Biela, and the 

 only result was a beautiful and perfectly harmless display 

 of meteors. A decrease in the heating power of the sun 

 seems to be the next most probable circumstance from 

 which we might fear the extinction of life on the earth. 

 But calculations founded on reasonable physical data show 

 that no appreciable ciiange can be going on, and experi- 

 mental data to indicate a change are wholly wanting. 

 Geological investigations sliow indeed that there have been 

 extensive variations of climate in past times ; vast glaciers 

 and icebergs have swept over the temperate regions at one 

 time, and tropical vegetation has nourished near the poles 

 at another time. I3ut here again the vicissitudes of climate 

 assume a periodic character, so that the stability of the 

 earth's condition does not seem to be threatened. 



All these statements may be reasonable, but they do not 

 establish the Uniformity of Nature in the sense that exten- 

 sive alterations or sudden catastrophes are impossible. In 

 tlie first place, Laplace's theory of the stability of the 

 planetary system is of an absti-act character, as paying 

 reji-ard to nothing but the mutual gravitation of the 

 ])lanetary bodies and the sun. It overlooks several 

 physical" causes of change and decay in the system which 

 were not so well known in his day as at present, and it also 

 presupposes the absence of any interruption of the course 

 of things by conflict with foreign astronomical bodies. 



It is now acknowledged by astronomers that there are at 

 least two ways in which tlie vis viva of the planets and 

 satellites may sutler luss. The friction of the tides_ upon 

 the earth produces a small quantity of heat which is 

 radiated into space, and this loss of energy must result in a 



